Digital cameras (or electronic cameras) with the function of tracking moving subjects are known. Even if a subject desired by a user moves, such digital cameras can track the subject and maintaining the focus.
As an example of the related art, an automatic tracking device is known that is capable of tracking an object in a stable manner while zooming in to and out from the object at high speed, where failure in the automatic tracking due to the zooming process is resolved. Such an automatic tracking device includes a first object detector, a first zoom controller, a second object detector, a template matching unit, and a second zoom controller. The first object detector detects an object from an image captured by the camera. The first zoom controller changes the zoom magnification of the camera when the size of the detected object is smaller than a specified size on an image. The second object detector detects an object again from an image captured by the camera after the zoom magnification is changed. The template matching unit compares the object detected by the second object detector with the image captured by the camera to locate the position of the object, thereby tracking the object on the image. The second zoom controller controls the zoom magnification of the camera such that the object being tracked will be in a specified size on the image captured by the camera. (For example, see Japanese Laid-open Patent Publication No. 2007-208453.)
As another example of the related art, a moving object tracking device which includes a camera, a first memory, a second memory, and a correlator is known. The camera includes a zooming mechanism. The first memory stores image signals which are sequentially input from the camera. The second memory extracts the image of a specified tracking object from the first memory, and stores the extracted image. The correlator extracts from the images in the first memory an image which is most similar to the image in the second memory. The moving object tracking device stores the image extracted by the correlator in the second memory as a new image of the tracking object, and controls the angle and zooming mechanism of the camera according to a difference between the currently stored image and the previously stored image. (For example, see Japanese Laid-open Patent Publication No. 59-079868.)
As another example of the related art, an image transmission system is known which enables a mobile station device to display moving images or continuous images which are partly selected from the captured video images taken by a video camera. This image transmission system includes: a video camera provided with a control drive device; a camera controller which detects the face of a subject animal from the data of the captured image to generate a control signal which controls the video camera to track the face of the subject animal; an image-data editor which modifies and edits the data of the captured image; and a transmission system which transmits continuous images, which are partly selected from the data of the captured image by the image-data editor, or moving image data to a mobile station device. The image-data editor converts the image obtained by extracting a face portion of the subject animal from the data of the captured image into an image with an approximately constant number of pixels and constant size, and then transmits the converted image. (For example, see Japanese Laid-open Patent Publication No. 2003-319386.)
In the object tracking where a subject is tracked by using images in a digital camera, a color region that matches the subject in an image frame is extracted for example. Here, in order to extract a target subject from the image frame according to the color components, a procedure is used in which whether or not the pixel values of neighboring pixels are the same as those of a target pixel is repeatedly determined.
On the other hand, real-time tracking is used for a digital camera that performs auto-focus control by using a result of object tracking. In other words, it is preferred that the processing time taken for extracting a region that corresponds to a target subject from the image frames be shortened.
However, when a target subject is large on an image frame, the amount of processing needed for extracting regions that correspond to the target subject becomes large. In other words, the processing time needed for object tracking becomes longer depending on the state of the target subject in an image frame.